A conventional electric coil device is designed to include multiple coil modules that are mounted to a block for achieving the functions of a transformer and an electronic filter. Each coil module includes two winding cores and four wires. The four wires are wound on one of the winding cores for achieving the function of a transformer. Two of the four wires are further wound on the other of the winding cores for achieving the function of a common-mode choke.
However, the structure of the two wires respectively wound on winding cores that provide dual functions of both a transformer and a common-mode choke is formed by first winding one of the wires on a corresponding one of the winding cores, followed by winding the other of the wires on the remaining one of the winding cores. This is due to difficulty or inconvenience in simultaneous winding of the two wires on the winding cores, and results in a slow and tedious winding process.